In the past, an internal combustion engine which has an exhaust variable valve timing mechanism which can change the phase angle (valve timing) of an exhaust valve has been known. In this internal combustion engine, for example, at the time of cold start of the internal combustion engine, control is performed for advancing the closing timing of the exhaust valve with respect to intake top dead center for the purpose of decreasing the emission of unburned hydrocarbons (unburned HC) from the engine body (for example, Japanese Patent Publication (A) No. 2003-120348).
That is, at the time of engine startup, not all of the fuel which is injected from a fuel injector is burned. Part of the fuel remains as unburned HC in the combustion gas. Here, in the system described in Japanese Patent Publication (A) No. 2003-120348, when in a state where combustion gas cannot be sufficiently again sucked into a combustion chamber due to the relationship of the pressure inside the intake passage and the pressure inside the exhaust passage, the closing timing of the exhaust valve is advanced. Due to this, even after the exhaust valve closing, a large amount of combustion gas remains in the combustion chamber. This combustion gas is sucked into the combustion chamber in the intake stroke of the next cycle. For this reason, a large amount of combustion gas is again taken into the combustion chamber. Accordingly, the unburned HC contained in this combustion gas will be burned in the next cycle. Due to this, it is possible to decrease the unburned HC contained in the exhaust gas.
In this regard, as control performed in an internal combustion engine which has an exhaust variable valve timing mechanism able to change the operating angle of an exhaust valve, there are various control routines other than the control described in the above Japanese Patent Publication (A) No. 2003-120348. As one of such control routines, an early exhaust valve closing control at the time of engine low load operation etc. may be mentioned.
In this early exhaust valve closing control, at the time of engine low load operation etc., the closing timing of the exhaust valve is advanced compared with the ordinary time and is at the advanced side from intake top dead center. Due to this, the exhaust valve is closed before the piston reaches top dead center. For this reason, the piston rises after exhaust valve closing, whereupon the combustion gas remaining inside the combustion chamber is compressed. Therefore, at the time of opening of the intake valve, the pressure of the exhaust gas inside the combustion chamber becomes high and at least part of the exhaust gas inside the combustion chamber flows back into the intake port. Due to such backflow of the exhaust gas, the fuel which was deposited on the wall of the intake port can be blown off, the atomization of the fuel which was injected from the fuel injector can be promoted, and the combustion state of the internal combustion engine can be improved.
In this regard, when the exhaust variable valve timing mechanism cannot change the operating angle (i.e., the opening time) of the exhaust valve, a drop in the indicated thermal efficiency of the internal combustion engine is invited. That is, when it is not possible to change the operating angle of the exhaust valve, advance of the closing timing of the exhaust valve leads to advance of the opening timing of the exhaust valve. If the opening timing of the exhaust valve is advanced at the advanced side of the expansion bottom dead center, the period during which the combustion gas causes the piston to be pushed down becomes shorter. As a result, the energy of the combustion gas can no longer be sufficiently converted to kinetic energy of the piston and a drop in the output torque from the internal combustion engine is invited. In particular, the effect becomes remarkable at the time of a deceleration operation of the internal combustion engine. Torque shock occurs due to the rapid drop of the output torque or the output torque falls too much and engine stalling is invited in some cases.